1. Field of the Invention
The invention relates to isocyanate-free foamable mixtures comprising prepolymers and a hydrocarbon blowing agent.
2. Description of the Related Art
Sprayable in-situ foams are employed for filling hollow spaces, in particular in the building sector. Here, they are used, inter alia, for sealing joins, e.g. around windows and doors, and act as excellent insulating materials so as to give good thermal insulation. Further applications are, for example, insulation of pipes or filling hollow spaces in industrial appliances with foam.
All conventional in-situ foams are polyurethane foams (PU foams) which in the uncrosslinked state comprise prepolymers which have a high concentration of free isocyanate groups. These isocyanate groups are able to undergo addition reactions with suitable reaction partners even at room temperature, as a result of which curing of the spray foam is achieved after application. The foam structure is produced by incorporating a volatile blowing agent into the as yet uncrosslinked raw material and/or by means of carbon dioxide formed by reaction of isocyanates with water. The foam is generally ejected from pressure cans by means of the autogenous pressure of the blowing agent.
Reaction partners employed for the isocyanates are alcohols having two or more OH groups, especially branched and unbranched polyols, or else water. The latter reacts with isocyanates to liberate carbon dioxide, as mentioned above, and form primary amines which can then add directly onto a further, as yet unconsumed isocyanate group. This results in formation of urethane and urea units which, owing to their high polarity and their ability to form hydrogen bonds in the cured material, can form partially crystalline substructures and thus lead to foams having a high hardness, pressure resistance and ultimate tensile strength.
Blowing agents used are mostly gases which are condensable at a relatively low pressure and can thus be mixed in the liquid state into the prepolymer mixture without the spray cans having to be subjected to excessively high pressures. In addition, the prepolymer mixtures contain further additives such as foam stabilizers, emulsifiers, flame retardants, plasticizers and catalysts. The latter are usually organic tin(IV) compounds or tertiary amines. However, iron(III) complexes, for example, are also suitable here.
PU spray foams are produced both as one-component (1K) foams and as two-component (2K) foams. The 1K foams cure exclusively through contact of the isocyanate-containing prepolymer mixture with atmospheric moisture. Foam formation can additionally be aided by the carbon dioxide liberated during the curing reaction of 1K foams. 2K foams comprise an isocyanate component and a polyol component which have to be mixed well with one another immediately before foaming and cure as a result of the reaction of the polyol with the isocyanates. An advantage of the 2K systems is an extremely short curing time of sometimes only a few minutes to achieve complete curing. However, they have the disadvantage that they require a complicated pressure can having two chambers and, in addition, are significantly less comfortable to handle than the 1K systems.
The cured PU foams display, in particular, excellent mechanical and thermal insulation properties. Furthermore, they have very good adhesion to most substrates and are stable virtually indefinitely under dry and UV-protected conditions. Further advantages are the toxicological acceptability of the cured foams from the point in time at which all isocyanate units have reacted quantitatively, and their swift curing and their easy handleability. Due to these properties, PU foams have been found to be very useful in industrial practice.
However, PU spray foams have the critical disadvantage that the isocyanate groups can, owing to their high reactivity, also develop a serious irritant action and toxic effects. In addition, the amines which can be formed by reaction of monomeric diisocyanates with an excess of water are in many cases suspected of being carcinogenic. Such monomeric diisocyanates are likewise present in addition to the isocyanate-terminated prepolymers in most spray foam mixtures. The uncrosslinked spray foam compositions are thus toxicologically unacceptable until they are completely cured. Critical factors here are not only direct contact of the prepolymer mixture with the skin but also, in particular, possible aerosol formation during application of the foam or vaporization of low molecular weight constituents, e.g. monomeric isocyanates. This results in the risk of toxico logically unacceptable compounds being taken up via inhaled air. In addition, isocyanates have a considerable allergenic potential and can, inter alia, trigger asthma attacks. These risks are increased by the factor that the PU spray foams are often not used by trained and practiced users but by handymen and home workers, so that correct handling cannot always be assumed.
The hazard potential exhibited by conventional PU foams and the associated compulsory labeling has additionally resulted in the problem of considerably decreasing acceptance of the corresponding products by users. In addition, completely or partly emptied spray cans are classified as hazardous waste and have to be labeled accordingly and in some countries, e.g. Germany, even have to be made available for reuse by means of a costly recycling system.
In order to overcome these disadvantages, DE-A-43 03 848, inter alia, has prescribed prepolymers for spray foams which contain no monomeric isocyanates or contain only low concentrations of these. However, a disadvantage of such systems is the fact that the prepolymers always still have isocyanate groups, so that such PU spray foams may well be better than conventional foams from a toxicological point of view but cannot be described as nonhazardous. In addition, the acceptance and waste problems are not solved by such foam systems.
It would therefore be desirable to have prepolymers which do not crosslink via isocyanate groups and are thus toxicologically acceptable available for the production of spray foams. Moreover, these prepolymer mixtures should make it possible to produce spray foams which in the cured state have similarly good properties and, in particular, a comparable hardness compared to conventional isocyanate-containing PU foams. In addition, one-component spray foam systems which cure exclusively through contact with atmospheric moisture also have to be possible. These should display comparably problem-free handling and processibility including a high curing rate even at a low catalyst concentration. The latter is important particularly since the organotin compounds generally used as catalysts are likewise problematical from a toxicological point of view. In addition, tin catalysts often also contain traces of highly toxic tributyltin derivatives. It would therefore be particularly advantageous to have a prepolymer system which has such favorable curing properties that a tin catalyst can be entirely dispensed with.
On this subject, the literature, e.g. U.S. Pat. No. 6,020,389, describes condensation-crosslinking silicone foams which comprise alkoxy-, acyloxy- or oximo-terminated silicone prepolymers. Such foamable mixtures are in principle suitable for producing 1K foams which cure at room temperature only through atmospheric moisture. However, such systems comprising purely silicone-containing prepolymers can be used only for producing elastic flexible to semi-rigid foams. They are not suitable for producing rigid, nonbrittle in-situ foams.
WO 00/04069 and WO 02/068419 describe prepolymer mixtures comprising alkoxysilane-terminated prepolymers for producing rigid spray foams. These are polymers having an organic backbone which generally has a conventional polyurethane structure. This can be formed by reaction of customary diisocyanates with polyols. If an appropriate excess of diisocyanates is used in this first reaction step, isocyanate-terminated prepolymers are obtained. These can then be reacted with aminopropyltrimethoxysilane derivatives in a second reaction step to form the desired alkoxysilane-terminated polyurethane prepolymers. In WO 02/068419, a specific reactive diluent is additionally added to this silane-terminated prepolymer. The prepolymers and any reactive diluents present can condense with one another in the presence of a suitable catalyst and of water with elimination of methanol and as a result cure. The water can be added as such or can originate from contact with atmospheric moisture. Both 1K and 2K foams can thus be produced using such a system.
However, the alkoxysilane-terminated polyurethane prepolymers described in WO 00/04069 and WO 02/068491 have, inter alia, the disadvantage of a relatively low reactivity toward atmospheric moisture. For this reason, high concentrations of a tin catalyst are necessary to achieve sufficiently rapid curing.
A significant improvement is provided by a system described in WO 02/066532. The alkoxysilane-terminated prepolymers described here for producing isocyanate-free spray foams comprise silane end groups of the general formula [1],
where:    X and Y are each an oxygen atom, an N—R3 group or a sulfur atom,    R1 is an alkyl, cycloalkyl, alkenyl or aryl radical having 1-10 carbon atoms,    R2 is an alkyl radical having 1-2 carbon atoms or an ω-oxaalkylalkyl radical having a total of 2-10 carbon atoms,    R3 is a hydrogen atom, an alkyl, alkenyl or aryl radical having 1-10 carbon atoms or a —CH2—SiR1z (OR2)3-z group and    z is 0 or 1,with the proviso that at least one of the two groups X and Y is an NH function.
In these alkoxysilyl-terminated prepolymers, the crosslinkable alkoxysilyl groups are separated from a urethane or urea unit by only a methyl spacer. These prepolymers are astonishingly reactive toward water and thus have extremely short tack-free times in the presence of atmospheric moisture and can even be crosslinked in the absence of tin.
A further critical disadvantage of silane-terminated prepolymers for spray foam applications could, on the other hand, be overcome neither in WO 00/04069 or WO 02/068491 nor in WO 02/066532. All silane-terminated prepolymers specifically described therein are largely incompatible with blowing agent mixtures consisting exclusively of or containing large proportions of hydrocarbons, e.g. propane/butane mixtures. Although emulsions which are stable for a number of hours and can also be foamed without problems can be prepared from the prepolymers and these blowing agents, these emulsions display substantial demixing on standing for a number of days or weeks. However, such long times left standing occur regularly in the practicable use of spray foam cans. Owing to the high viscosity of the silane-terminated prepolymers, reemulsification after demixing can only be achieved by extremely vigorous and long shaking. In general, the prepolymer is so highly viscous that reemulsification of the prepolymer/blowing agent mixture at room temperature is no longer possible at all, and the emulsion additionally has to be heated to >40° C. before shaking. However, such complicated procedures prior to processing of spray foams would no longer be acceptable to the users of spray foams.
Although the prepolymers described in WO 00/04069, WO 02/068491 or WO 02/066532 display sufficient compatibility with other blowing agents, these blowing agents each have other critical disadvantages. Thus, dimethyl ether displays a considerable destabilizing action on the as yet uncured foam. Dimethyl ether contents of >20% in the blowing agent mixture are therefore problematical or, in most cases, even completely impossible. On the other hand, fluorine-containing blowing gases are regarded as critical because of their action as greenhouse gases and are already prohibited for spray foam applications in some countries, e.g. Denmark.
In particular, however, all polar blowing agents, i.e. all fluorinated blowing gases which are commercially available at a favorable price and also dimethyl ether, have the disadvantage that after foaming they can slowly diffuse through the as yet uncured foam lamellae. This diffusion of a polar blowing agent through the foam lamellae composed of likewise polar material proceeds significantly more quickly than the diffusion of nonpolar air occurring in the opposite direction. This can lead to shrinkage of the foam and may even lead to crack formation, since, unlike in the case of conventional PU foams, no carbon dioxide is liberated during curing and is able to compensate the blowing agent shrinkage. The shrinkage and the associated crack formation make the use of such foams in joins having an unfavorable geometry impossible.